1. Field of the Invention
The present invention relates to an engine valve formed of titanium alloy used for an internal combustion engine, the engine used, for example, in an automotive vehicle. The present invention also relates to a method of making the engine valve, and to a cylinder head incorporating the valve.
2. Description of the Background Art
Hitherto, in order to improve a performance of an engine, attempts have been made to increase the engine's usable rotary speed range. The largest obstacle for enhancing the permissible number of engine revolutions over a period of time is an increase in inertial mass of a valve system. In other words, an increase in the inertial mass of the valve system causes reduced capability of an engine valve to follow a cam as the engine rotary speed is increased, which may result in reducing engine output or the like. Therefore, in order to achieve an increase in engine rotary speed, reduction of the inertial mass of the valve system is important. In particular, reduction of the weight of the engine valves, such as an air-intake valve or an exhaust valve, which occupy a large portion of the mass of the components of the valve system, is extremely effective for increasing the engine rotary speed.
Therefore, recently, in order to reduce the weight of the engine valve, engine valves have been formed of a titanium alloy which is low in weight and high in strength, instead of heat-resisting steel used in the related art. For example, in JP-A-2001-234313, a technology to manufacture an engine valve formed of titanium alloy is disclosed. The disclosed engine valve includes a stem section and a bevel section formed integrally to each other by hot-forging. More specifically, in JP-A-2001-234313, a method is proposed in which a bevel section is formed of a needle-like (acicular) α crystallographic texture at an end of the stem section by forming an enlarged diameter section at an end of the stem section formed of an equiaxed a crystallographic texture at a temperature exceeding a β transformation point, and then immediately hot-forging the enlarged diameter section.
In the manufacturing method described in JP-A-2001-234313, the enlarged diameter section, which is formed into the needle-like texture, is formed by upset forging the end of a shaft-shaped member with an electric setting machine, and then immediately forging the enlarged diameter section. As a result, the following problems occur: Shrinkage may easily occur at the enlarged diameter section at the time of upset, and forging defects such as shrinkage, cracking, or entrainment may also easily occur on the formed bevel section when forging the enlarged diameter section.
In the engine valve manufactured by this manufacturing method, a boundary is formed between the above-described needle-like texture and the equiaxed texture at a boundary (neck portion) between the stem section and the bevel section, where a stress is intensively applied due to its shape. As a result, the strength at the neck portion may be lowered. In order to secure the strength of the neck portion, where a stress is intensively applied, a countermeasure such as enlarging the diameter of the neck portion is necessary. However, such enlargement results in increase in weight of the engine valve.
In addition, in the method of manufacturing the engine valve, since a positional accuracy of the boundary between the needle-like texture and the equiaxed texture is not high, an increased in the variation in strength of components of the manufactured engine valve occurs.
Therefore, the present invention is proposed in view of the above described circumstances in the related art, and it is an object of the invention is to provide an engine valve formed of titanium alloy which further improves manufacturing reliability and productivity while improving valve creep resistance and a fatigue strength at a high temperature.